Propelled by light: the promise and perils of solar sailing

Earlier this week, the Planetary Society, a space advocacy group in Pasadena, California, received an anonymous donation to build and launch a small solar-sail driven spacecraft.

The Society hopes to launch the sail in about a year as part of a three-stage plan to demonstrate the viability of solar sail propulsion, which has never been tested in orbit. The group says it is the only practical technology that might be used for interstellar travel, since the light generates a small but constant pressure that should accelerate a sail to high speeds over time.

New Scientist caught up with Louis Friedman, the organisation's executive director, to find out more about the promise and challenge of solar sailing.

What is a solar sail and how does it work?

A solar sail is a device that collects sunlight and transfers the energy of the sunlight to the momentum of the spacecraft. It uses pure light, reflecting off the sail, so you want a large area to collect a lot of photons and you want it highly reflective so you get a high efficiency of them bouncing off. We use aluminised mylar.

What's the history of the idea?

The pressure of sunlight was noted by the early pioneers who discovered light. Felix Tisserand and others noted how light pressure affected comet [tails] back in the 19th century. The use of light to propel a spacecraft, that idea was invented by Fridrich Tsander and Konstantin Tsiolkovsky back in the Soviet Union in the 1920s. But it wasn't until the 1970s that anyone thought about practically doing it. Solar pressure force has been measured on spacecraft many times and it's been used in manoeuvres, but never as the single force to propel your way around in space.

What kind of missions are solar sails good for?

It's spacecraft without fuel, and the real advantage comes when you go really long distances. The ultimate goal is interstellar travel because this is the only known technology that can be used over those kinds of distances.

You need ambitious missions in the long run, but there's a lot more interest in using sails right now for missions that hover between Earth and the sun to monitor solar weather, or that do a pole-sitting above the Earth to monitor the arctic environment on the land and the magnetosphere.

We eventually want to put a solar weather station at the L1 point between Earth and the sun [Lagrange points are regions of space where the gravitational acceleration from the Earth and sun are exactly equal, letting objects remain there with very little effort]. This could give us longer warning times for coronal mass ejections and solar storms that impact the ionosphere and can cause disruptions in the power grid.

What is the plan for the society's 'Light Sail'?

We have a three-step process: LightSail-1 will prove solar sailing, LightSail-2 should be able to use the sail to increase orbital energy, getting away from Earth into higher orbit to do scientific measurements. LightSail-3 would then go all the way out to become the first step of the solar-weather monitoring station.

Because we're doing it with this very new technology, Nanosats, it becomes affordable. These are tiny satellites that, if you had to bring them to the launch site, you wouldn't check them in your luggage – you'd just put them in the overhead compartment.

Space agencies don't want to invest in a big spacecraft with a whole new technology with large structures and deployable devices without it being proven. But, of course, the only way to prove it is to do it, so it becomes a catch-22. What we're doing now with the Nanosats is finding an affordable way to prove it. The hardware costs are very minimal.

What are the challenges of flying a solar sail?

What's uncertain, and what becomes difficult to model until we get the craft into space itself, are the dynamics – the fluttering and shaking, how it reacts to micrometeorites, stuff like that. Even turning it might set up some resonant dynamic motions because it's so long and so thin – that's a big uncertainty. That's why we build this up step-by-step. Our first flight is just to show that we can control the vehicle, that we can turn it without any bad things happening. Based on that experience, we'll build it larger and then move on to the second step.

How big is the current craft?

The sail is 32 square metres when it's unfurled, 5.5 m on a side. The mass of the craft is 4.5 kilograms. [For interstellar travel], I would estimate you'd need a sail that's half a kilometre by half a kilometre in area and a craft that's 1 kilogram in mass. And instead of a 5-micron sail like we have, which is very thin, with really advanced technology you might have a 1-micron sail by the time you do interstellar travel. This would also need a high-powered laser to keep enough light energy focused on the sail.

One kilogram isn't much mass – it sounds like humans would be too heavy for interstellar solar sail missions.

Yeah, I don't think human interstellar travel ever will be done by sailing. I already consider myself way-out for talking about interstellar flight for spacecraft, I don't know how to talk about human interstellar flight.

So what could be learned by sending probes out of the solar system?

If you think about the evolution of everything we're doing, it's all about sending information to places and from places. The mass of the physical things we send is getting very small and the content of what we're learning gets very large.

I envision giant sails doubling their duty as giant antennas, collecting information and beaming it back with incredible algorithms of information compression – and other tricks that haven't been invented yet – that provide lots of information about other star systems. I think they will extend our outreach of exploration into the stars.

What got you interested in solar sails?

That goes back to the 1970s, when I was working at JPL [NASA's Jet Propulsion Lab]. Solar sailing was going to be the way that we could do something that we thought could never be done, which is rendezvous with Halley's Comet. The comet comes into the solar system backwards, basically. To meet up with it, you have to stop the world and get off – you have to cancel out all of your orbital motion around the sun, turn completely around, go the other way and then come in and rendezvous with the comet. We thought that could never be done, and then we found a solar sail design that could do it. I got really excited – that's what excites me, doing things that have never been done before.

That mission turned out not to be ready in time, but a lot of people wanted to continue with solar sailing, as did I, but I didn't want to spend time on it unless I had a flight opportunity. It wasn't until the Russians came to us in 1999 and said [they could launch] our Cosmos 1 mission that I got energised again. And now the Nanosats have made me even more energised because they're going to open up the universe. They're going to open us up to all kinds of ways of doing things in space because they're affordable. This is a low-cost project, a couple of million dollars, and at the same time very ambitious.

Was it hard to bounce back after the Society's Cosmos 1 solar sail crashed into the sea after launch in 2005?

Yes and no. First of all, I was incredibly uplifted because so many people were so positive. That's what you do in the space business, you have failures and then you bounce back. You're at the edge of it. So yes, it was relatively easy to bounce back. At the same time, you know, you don't want to keep on doing the same thing. You learn from a bad experience and you do the next mission even better.

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Solar sails can pick up a lot of speed, thanks to the constant pressure from sunlight. In the future, they might be able to traverse the solar system in the span of just a few years (Illustration: Rick Sternbach/Planetary Society)